Replacement of tendons is a problem not yet satisfactorily solved because tendons derived from another individual are rejected. Sometimes a less necessary tendon from the same individual can be used, but this can hardly be considered an ideal solution. There was, until now, no synthetic material available possessing at the same time, physiological inertness, hydrophilicity and permeability for solutes together with high tenacity and elasticity. The usual synthetic filaments such as aromatic polyesters or polyamides possess, in a suitable arrangement of a set of twisted yarns good tenacity and elasticity, they are, however, impermeable for dissolved metabolites and irons; This property may increase the tendency towards some diseases, particularly if the size of the prosthesis is large. Synthetic hydrogels with proved compatibility with living tissues having usually a too low tensile strength.
According to the invention. The tendon prosthesis consists of two parts with very different characteristics. The core bearing the load is formed from a mono-axially oriented linear polymer having the desired strength and a suitable elastic elongation of a value similar to that of a genuine tendon. The elastic elongation is obtained by imparting to the yarn a suitable twist. Instead of a yarn or cord a knitwork can be used, constructed in such a way that the average angle between individual filament and the load axis is less than 35.degree., as is twisted yarn. It is, however, possible to use hydrophilic linear polymers having sufficient elasticity as such, without any twist or knitting. Said modes can be combined, e.g. by using a fiber bundle having a genuine elastic elongation of 7 percent and twisting it so that the average angle of orientation is 12.degree., whereby an overall elastic elongation of 10.5 percent of the rest length is obtained.
Another way to obtain the desired overall elastic elongation is by using two materials: The central part of the core can be made from an elastic oriented hydrophilic polymer having a 20 percent elastic elongation while the outer layer of the core is a tubular knitwork made from a synthetic filament having a 1 percent elastic elongation, the individual filaments including with the load axis an average angle of 23.degree., Then, the core as a whole has a 20 percent elastic elongation.
Combinations of two or more materials of the core are advantageous because they make it possible to obtain desirable shape of the stress-strain curve, or that of the stress-modulus curve, with optimum characteristics from the view-point of the function of the prostheses. Such combinations also make possible a reliable fastening by means of a suture because the less elastic component protects the more elastic one against cutting through; moreover it makes it possible to fix the prosthesis not only by sewing but also by means of suitable adhesives, leaving the ends of the less elastic component such as the textile fabric or knitwork reinforcement protruding from the prosthesis. Oriented cords or yarns can protrude from one end or from both ends in the form of loops which can be easily fastened.